1. Field of the Invention
This invention relates to a process for the production of acrylic acid by the catalytic vapor-phase oxidation of propylene. More particularly, it relates to a process for highly efficient production of acrylic acid owing to the use of a raw gas of a specific composition.
2. Description of the Prior Art
The process for the production of acrylic acid by the two-stage (the first stage reaction for conversion of propylene mainly to acrolein and the second stage reaction for conversion of acrolein mainly to acrylic acid) catalytic vapor-phase oxidation of propylene has been known to the art. Any attempt at improving the productivity of this process in the formation of acrylic acid has been blamed for the following drawbacks: (1) The space velocity cannot be appreciably increased on account of the limit imposed on the quality of a catalyst (yield and service life) to be used. (2) The conversion of propylene cannot be enhanced without lowering the selectivity of the reaction for reaction products favoring the production aimed at. (3) The room for addition to the propylene content of the raw gas is limited because the removal of the heat of reaction and the protection of the reaction system against the danger of explosion pose themselves as serious problems.
In the circumstances, various studies have been made in search of a way of improving the productivity of a process employed for the production of acrylic acid. For example, a process which comprises recycling unaltered propylene for reuse thereby increasing the conversion of propylene is disclosed in G.B. Patent No. 996,898. Numerous publications regarding the reclamation of reaction off-gas leaving the reactor are found in literature. For example, U.S. Pat. No. 3,540,201 discloses a process which comprises varying the activity of a catalyst in two stages, increasing the activity continuously or gradually up to 100% from the entrance to the reaction tube along the length of the reaction tube, removing most of the condensible gas at the outlet of the second-stage reaction zone, and resupplying the reaction off-gas remaining after the removal of the condensible gas in the place of part or whole of steam to the first-stage reaction zone. This process entails a work of diluting the catalyst and consequently suffers from deficiency practicability. It also exhibits poor and hardly tolerable productivity because of the use of propylene at a low concentration.
U.S. Pat. No. 4,031,135 discloses a process which curbs the post-combustion by utilizing the returned off-gas. This returned off-gas substantially comprises nitrogen and additionally contains small amount of unaltered propylene, oxygen, propane, and carbon oxides.
G.B. Patent No. 1,450,986 discloses an invention which concerns use of carbon dioxide formed in a reaction system. The use of the carbon dioxide as the carrier gas is claimed to prevent the danger of explosion, facilitate the removal of the heat of reaction from the reaction bed, and contribute to heightening the yield. This patent, however, has no specific disclosure about a process for the production of acrylic acid by the oxidation of propylene.
U.S. Pat. No. 4,052,450 discloses a process which comprises subjecting an .alpha.-olefin to oxidation with molecular oxygen or to ammoxidation in the vapor-phase by the use of a catalyst containing small amounts of indium and/or aluminum and/or lanthanum and/or gallium in the form of oxides and/or mixed oxides and containing molybdenum and bismuth and optionally other elements in the form of oxides or mixed oxides at a temperature in the range of 280.degree. to 450.degree. C., when necessary, in the presence of steam and/or ammonia thereby producing a corresponding .alpha., .beta.-olefinically unsaturated aldehyde or nitrile. In this specification, there is a mention purporting that the content of the .alpha.-olefin such as propylene or isobutylene in the synthetic raw gas generally falls in the range of 0.5 to 15% by volume, particularly in the range of 2 to 6% by volume when oxygen is used in place of air, and in the range of 6 to 15% by volume when pure oxygen is used and part of the reaction off-gas remaining after removal of acrylic acid is added (by recycling) to the gaseous mixture subjected to the oxidation. The concentration of oxygen generally falls in the range of 2 to 20% by volume, preferably 5 to 15% by volume. The mixture further comprises carbon monoxide, carbon dioxide, and nitrogen and additionally contains, mostly in small amounts, rare gases, hydrogen, ethylene, and propane. The synthetic raw gas may contain steam. There is a mention purporting that the content of steam generally falls below 40% by volume, desirably below 20% by volume and particularly below 10% by volume, and particularly preferably in the range of 2 to 8% by volume. In this patent publication, however, no specific mention is made of production of acrylic acid by oxidation of propylene.
Japanese Patent Publication SHO 39(1964)-3,670 discloses a process which uses pure oxygen as an oxygen source, withdraws a circulation gas formed of unaltered propylene, replenishes the circulation gas with fresh supply of propylene, oxygen, and steam, returns the refreshed circulation gas to the reactor, and uses as a diluent gas the CO.sub.2 and CO produced during the reaction and steam.
G.B. Patent No. 1,390,271 discloses a process which withdraws a carbonyl compound and a carboxylic acid compound while pure oxygen is used as an oxidizing agent, treats the produced mixture remaining after the withdrawal for removal of carbon dioxide, and circulates to the reactor the residual mixture containing unaltered olefin and oxygen in combination with water.
G.B. Patent No. 1,220,568 discloses use of pure oxygen as an oxidizing agent. As a diluent for propylene and oxygen, the off-gas from the second stage reaction and remaining after separation of acrylic acid is used. This diluent contains carbon oxides and steam as inactive components. According to the preferred embodiments cited in the specification, the oxygen:propylene ratio is substantially less than 1.5:1. Thus, this process has the possibility of disposing the catalyst to reduction due to insufficient supply of oxygen, lowering the selectivity of the reaction for acrolein and acrylic acid and the conversion of propylene and impairing the service life of the catalyst.
The improvement of the productivity of a process for the manufacture of acrylic acid, as described above, is achieved by increasing the amount of propylene to be supplied as a raw material and minimizing the load upon the catalyst by shortening the contact time of the raw gas on the catalyst bed to the fullest possible extent. An increase in the amount of propylene subjected to the reaction (with a simultaneous elevation of the space velocity) results in an increase in the volume of heat generated by the reaction of oxidation. An attempt at increasing the conversion of propylene entails an inevitable increase in the oxygen concentration of the raw gas. The increased oxygen concentration renders it difficult to prevent propylene from reaching a range of explosion. Heretofore, for the removal of a large volume of heat and the protection of the reaction system against the danger of explosion, the forced entrainment of steam by the raw gas and the dilution of the portion of the catalyst abundantly participating in the reaction with an inactive carrier, for example, have been resorted to an effective measures. The decrease of the oxygen concentration in the raw gas by the addition of the recovered off-gas has also been employed as an effective measure. These measures, however, necessitate extra labor for the dilution of the catalyst, entail an inevitable decrease in the concentration of the recovered acrylic acid due to the addition of steam, and induces loss of energy due to the separation of acrylic acid. The conventional processes have failed to effect sufficient removal of the heat of reaction from the off-gas and have attained a propylene oxidation with a high propylene throughput only to a limited extent.
An object of this invention, therefore, is to provide a novel process for the production of acrylic acid by the catalytic vapor-phase oxidation of propylene.
Another object of this invention is to provide a safety oxidation process with a high productivity, in which production of acrylic acid is in a high yield for a long period of time even under reaction conditions to impose a high load on a catalyst.